TAXA OF LIFE
SUPERGROUP UNIKONTA

INTRODUCTION TO THE UNIKONTA

In 1993 Baldauf and Palmer demonstrated the phylogenetic relationship between the animals and fungi by using sequences of 25 proteins.  This followed a suggestion by Cavalier-Smith (1987) that animals and fungi evolved from a choanoflagellate-like line.  Since then, other work (Cavalier-Smith and Chao, 1995; and Cavalier-Smith, 1998b) strengthened the link and called the group, the Opisthokonts in reference to the posteriorly-directed flagellum in motile cells.  Other work (e.g. van de Peer et al., 2000; and Keeling, 2004) has confirmed a sister group relationship between the Amoebozoae and the Opisthokonts.  Tentatively, this group is called the Unikonta, in reference to the occurrence of a single flagellum in their motile cells.

The Unikont question hinges on the relationships of the Apusozoa with other taxa, especially the Amoebozoa.  Is Apusozoa a member of the Bikonts like the Excavata or the Unikonts?  Literally, because the Apusomonads have two flagella that are heterodynamic, they are Bikonts.  If that is where they really belong, then the Unikonta collapses, and the Opisthokonts and Amoebozoans would have to be considered separate supergroups.

Cavalier-Smith and Chao (2003; see Figure 1) present a scenario in which the unikonts, those organisms with single flagella, are paraphyletic.  They root the eukaryotes in a line from a group of Gram positive eubacteria, which gave rise to the eukaryotes and the archaea [see an explanation of the Archaezoa Hypothesis in the description of the Domain Eukarya].  The stages in the origin of the eukaryote line were the organization of microtubules to form a flagellum (not the endosymbiotic origin as Margulis claims) and the later acquisition of a mitochondrial endosymbiont.  The single flagellar structure gave rise to the Opisthokonts (taxa with a single trailing or pushing flagellum and flat mitochondrial cristae) and the Anterokonts (taxa with a single anterior or pulling flagellum and tubular mitochondrial cristae).  The anterokonts (e.g. Amoebozoae) gave rise to the Bikont line, which they presumed to have arisen through an apusozoan-like organism.  

In a curious turnabout from Cavalier-Smith and Chao (2003), Cacalier-Smith (2003; Figure 2) presents a larger view of the Eukaryotes and suggests that the Amoebozoa is in the clade with the Opisthokonts.  In this scenario, all Eukaryotes can be segregated into Unikonta and Bikonta.  The Apusomonads are at the base of the Bikont line.

Kim et al. (2006; Figure 3) suggest a third alternative.  They conclude that the Apusozoans are sisters to the Opisthokonta and that the Amoebozoa are sisters to all other Bikont Eukaryotes.  In this scenario, the Unikonts collapse as monophyletic taxon.  

The Unikonts also suffer from a lack of defining synapomorphies.  Table 1 explores the distribution of states of flagella, cell covering, mitochondria, and meiosis through 12 groups of Unikonts.  The flagella range from being absent to one posterior, one anterior, and both posterior and anterior.  Similarly, mitochondria can be absent, reduced, or present with flat cristae or present with tubular cristae.  Character states for cell covering and meiosis are equally dispersed.  

LITERATURE CITED

Adl, S. M., A. G. B. Simpson, M. A. Farmer, R. A. Andersen, O. R. Anderson, J. R. Barta, S. S. Bowser, G. Brugerolle, R. A. Fensome, S. Fredericq, T. Y. James, S. Karpov, P. Kugrens, J. Krug, C. E. Lane, L. A. Lewis, J. Lodge, D. H. Lynn, D. G. Mann, R. M. McCourt, L. Mendoza, O. Moestrup, S. E. Mozley-Standridge, T. A. Nerad, C. A. Shearer, A. V. Smirnov, F. W. Spiegel, and M. F. J. R. Taylor. 2005. The new higher level classification of eukaryotes with emphasis on the taxonomy of protists. Journal of Eukaryotic Microbiology. 52(5):399-451. 

Baldauf, S. L. and J.D. Palmer. 1993. Animals and fungi are each other’s closest relatives: Congruent evidence from multiple proteins. Proceedings of the National Academy of Science USA. 90:11558-11562. 

Cavalier-Smith, T. 1987b. The origin of eukaryote and archaebacterial cells. Annals of the New York Academy of Sciences. 503: 17-54.

Cavalier-Smith, T. 1998b, Neomonada and the origin of animals and fungi: In: Coombs, G. H., Vickerman, K., Sleigh, M. A. & Warren, A. (ed.), Evolutionary Relationships Among Protozoa. Kluwer Academic Publishers. Dordrecht , The Netherlands , pp. 375–407.

Cavalier-Smith, T. 2003a. Protist phylogeny and the high-level classification of Protozoa. European Journal of Protistology. 39:338-348. 

Cavalier-Smith, T. and E. E. Chao. 1995. The opalozoan Apusomonas is related to the common ancestor of animals, fungi, and choanoflagellates. Proceedings of the Royal Society of London B. 261:1-6.

Cavalier-Smith, T. and E. E. Chao. 2003b. Phylogeny of Choanozoa, Apusozoa, and other Protozoa and early eukaryote megaevolution. Journal of Molecular Evolution. 56:540-563.

Keeling P. J. 2004 The diversity and evolutionary history of plastids and their hosts. American Journal of Botany. 91(10): 1481-1493. 

Van de Peer, Y., A. Ben Ali, and A. Meyer. 2000. Microsporidia: accumulating molecular evidence that a group of amitochondriate and suspectedly primitive eukaryotes are just curious fungi. Gene. 246: 1-8.